Conventional color flow imaging, including "angio" or "power Doppler imaging" (referred to hereinafter as "flow imaging"), produces one image from a sequence of transmitted pulses (a packet), typically in the range of 5-15 pulses for each scan line in the image. Slowly moving muscular tissue produces lower Doppler shift in the received signal than signal from moving blood, and efficient clutter filters are designed to suppress the clutter signal to a level much lower than the signal from blood. The signal power after clutter filtering is used to detect points in the image where blood is present. An alternative is to display the signal power as an image (angio or power Doppler) to visualize blood vessels. In order to get reliable detection, substantial temporal and spatial averaging is used, thus limiting the dynamic variation, as well as spatial resolution (bleeding). This averaging process suppresses the spatial speckle pattern in the signal amplitude.
Conventional ultrasound blood flow imaging is based on detection and measurement of the Doppler shift created by moving scatterers. This Doppler shift is utilized to suppress the signal from slowly moving muscular tissue, in order to detect the presence of blood, and is also used to quantify the actual blood velocity in each point of an ultrasound image. Unfortunately, the Doppler frequency shift is only sensitive to the velocity component along the ultrasonic beam; possible velocity components transverse to the beam are not detected or measurable from the received signal Doppler spectrum. In standard color flow imaging, the Doppler shift is estimated from the received signal generated by a number of transmitted pulses, and coded in a color scale. In some situations, the blood flow direction can be measured from the vessel geometry, but this is difficult to do in an automatic way, especially when the vessel geometry is not clearly visible in the image. Standard color flow imaging often gives confusing blood velocity visualization; e.g., in a curved blood vessel the Doppler shift, and therefore also the color, is changing along the vessel due to change in the angle between the blood velocities and the ultrasonic beam, even though the velocity magnitude is constant. In power Doppler (also called the angio mode) this problem is solved by discarding the measured Doppler shift from the display.
There is considerable interest in measuring the transverse velocity component in ultrasound flow imaging, and a number of methods have been proposed. Compound scanning from two different positions was disclosed by Fox in "Multiple crossed-beam ultrasound Doppler velocimetry," IEEE Trans. Sonics Ultrason., Vol. 25, pp. 281-286, 1978. Compound scanning from two different positions gives two velocity components, but there are practical problems with the large-aperture transducer, the time lag between the measurement of the two components, and the limited field of view. In accordance with a method disclosed by Newhouse et al. in "Ultrasound Doppler probing of flows transverse with respect to beam axis," IEEE Trans. Biomed. Eng., Vol. 34, pp. 779-789, October 1987, the transit time through the ultrasound beam is measured, which is reflected in an increased bandwidth of the Doppler signal. This method has very low accuracy, does not yield flow direction, and will only work in regions with rectilinear and laminar flow. Two-dimensional speckle tracking methods based on frame-to-frame correlation analysis have been proposed by Trahey et al. in "Angle independent ultrasonic detection of blood flow," IEEE Trans. Biomed. Eng., Vol. 34, pp. 965-967, December 1987. This method can be used both for the RF signal and the amplitude-detected signal. Coherent processing of two subapertures of the transducer to create lateral oscillations in the received beam pattern has been described by Jensen et al. in "A new method for estimation of velocity vectors," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., Vol. 45, pp. 837-851, May 1998, and by Anderson in "Multi-dimensional velocity estimation with ultrasound using spatial quadrature," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., Vol. 45, pp. 852-861, May 1998. This method gives quantitative lateral velocity information, including the sign. The main drawback of this method is poor lateral resolution, which limits its use for imaging.
There is a need for a method of ultrasound imaging which gives the system user a correct perception of the blood flow direction and magnitude, and which is also useful to separate true blood flow from wall motion artifacts.